For transmission of optical signals within the Synchronous Digital Hierarchy (SDH) corresponding to the European standard or within the Synchronous Optical Network (SONET) corresponding to the North American standard as the case may be, transmission methods that are as inexpensive as possible are sought. For cost reasons, glass fiber, in particular, is used as a transmission medium because of the large bandwidth and range due to the low attenuation. The optimum use of these optical connections is sought by multiple utilization of glass fibers using Wavelength-Division Multiplexing (WDM). To protect the transmission in the event of failure of a link, the signals are carried separately on two glass fibers (working and protection).
From patent application DE 19 731 494 of the applicant, a circuit arrangement is known whereby the protection of the transmission in ring structures is guaranteed by the multiple utilization of glass fibers using Wavelength-Division Multiplexing (WDM).
The corresponding application papers laid open to public inspection DE 19 731 494 describe a method of data transmission in a bi-directional working channel between several terminals of an optical ring network with a protection device, whereby in the event of a disturbed data transmission a protection connection using a wavelength-division multiplexing method in a working channel via the undisturbed section of the ring network is enabled, whereby a single bi-directional protection channel having a predetermined wavelength range, that has at least the transmission capacity of the working channel, connects all the terminals together, and in the event of a connection disturbance between the terminals adjacent to a disturbed section of the link, a protection connection is then established in the protection channel via the undisturbed section of the ring network, with the unaffected terminals being looped through the protection channel. In practice, this is achieved in that in the event of a fault of a working line assembly a switch to the assigned protection line assembly takes place in order to continue to maintain the working connection. In this case, the changeover takes place in the electronic assemblies.
Accordingly, an arrangement for data transmission between several terminals in an optical ring network with a protection device is also disclosed, that in the event of a fault enables data transmission in a working channel via the undisturbed section of the ring network using wavelength-division multiplexing, with wavelength-division demultiplexers via which wavelength-individual working signals and protection signals are decoupled, and wavelength-division multiplexers via which the wavelength individual working signals and protection signals are coupled, with a single bi-directional protection channel having a predetermined wavelength range with at least the transmission capacity of the working channel being provided that connects all terminals together.
The optical switches used here between the outputs of the wavelength-division demultiplers and the inputs of the wavelength-division multiplexers as well as of line assemblies, serve only to loop through the protection channel in the event of an undisturbed connection or in the event of a terminal affected by the fault to be able to switch it to the protection channel, to establish a protection connection via the undisturbed section of the ring network. The actual switching between both data transmission channels takes place in the electronic part of the network element.
The method described here is provided as an application in an optical ring network and the associated circuit arrangement is relatively expensive, therefore, it is not cost-effective for a pure “point-to-point” connection.
In WO 00/28670, an optical switch is used to switch between two transmission lines or two channel signals. However, in this case it is necessary to perform an optical monitoring before the optical switch for each channel of the two channel signals, via which the optical switch is controlled via the control unit.
In EP 0 920 153 A2, a version for a protective transmission of a ring network is disclosed. Two optical switches SW 618, 619, each with an output signal λ3, λ4, are placed in circuit before a receiver side network element TO NE. The input signals of the optical switches SW 618, 619 correspond in each case to a “wavelength-identical” output signal (e.g., λ3) of two demultiplexers DEMUX 700, 710 assigned, in each case, to an optical transmission line 101, 102, that from the WDM signal transmitted via the optical lines 101, 102 forms four output signals with different wavelengths λ1, λ2, λ3, λ4.
To switch through one of the two input signals to one of the optical switches SW 618, 619, a monitor unit is provided that is supplied with parts of both input signals and is correspondingly controlled via a monitoring channel λs.
An object of the present invention is, therefore, to provide a simple circuit arrangement whereby line protection with a “point-to-point” connection is achieved in the most inexpensive possible manner.